The present invention relates to free flowing silicone resin powder having a primary particle size in the range of about 0.1-200 nm and an aggregate size in the range of about 10 nm to 200 microns. More particularly, the present invention relates to the spray drying of an organic solvent dispersion of an organosiloxane hydrolyzate comprising chemically combined triorganosiloxy units and tetrasiloxy units to form a free-flowing silicone resin powder useful in making heat curable organopolysiloxane compositions.
A method for making silicone resin powder is shown by Wolfgruber et al, U.S. Pat. No. 4,935,484. Aqueous colloidal suspensions of resinous copolymer are spray dried to produce silicone resin powders having an average particle diameter of 10 to 150 nm. Although the spray dried powders of Wolfgruber et al can be utilized in a variety of applications as fillers, the powder still contains the emulsifying agent used in the preparation of the resinous copolymer as an impurity.
As taught by Wolfgruber et al, the silicone resin powder made by spray drying aqueous resinous copolymer can be used in plastics as a substitute for finely divided silica which has been rendered hydrophobic. However, it has been found that the aqueous spray dried powder made in accordance with the method of Wolfgruber et al having triorganosiloxy units chemically combined with tetrasiloxy units is incompatible with silicone fluids.
Attempts therefore to utilize the spray dried resinous copolymer powder made by the procedure of Wolfgruber et al to produce heat curable silicone compositions, such as pressure sensitive adhesives (PSA) shown by Murakami et al, U.S. Pat. No. 4,774,297 have been unsuccessful. It would be desirable therefore to provide a procedure for making finely divided silicone resin powder which is dispersible with silicone fluids and useful for making heat curable silicone compositions.